Notes from the Oesper Collections Hofmann Demonstration Apparatus William B. Jensen Department of Chemistry, University of Cincinnati Cincinnati, OH 45221-0172

The 19th-century German chemist, August Wilhelm Hofmann (figure 1), is well-known to historians of chemistry for his work in elucidating the chemistry of the amines, his early contributions to the synthesis of aniline dyes, and his discovery of the Hofmann degra- dation and elimination reactions. He also played a key role in the professional development of chemistry in both Great Britain and Germany – the former through his nearly 20-year directorship (1845-1865) of the Royal College of Chemistry in London (1) and the latter through his role, after his return to Germany, in the founding of the Deutsche chemische Gesellschaft and his service as that society’s first president (2). After his death, the society’s new headquarters were named the “Hofmannhaus” in his honor and the society commis- sioned a detailed book-length biography coauthored by no less a luminary than Emil Fischer (3). Less often commented on by his more recent biog- raphers is Hofmann’s interest in chemical education and his development of a large number of standard 19th-century chemical demonstrations. The present author has drawn attention to some of these in previous publications and lectures (4, 5). Indeed, these demon- strations were so popular that laboratory supply houses Figure 1. August Wilhelm Hofmann (1818-1892). soon began offering specially designed “Hofmann Demonstration Apparatus” for sale and several surviv- dent of the German Chemical Society, he gave a de- ing examples of this equipment are to be found in the tailed lecture on these newer developments to the soci- Jensen-Thomas Apparatus Collection. ety as part of a special meeting convened to celebrate Hofmann’s interest in lecture demonstrations be- the opening of his new chemical laboratory at the Uni- gan in 1860 with the publication of a brief paper on a versity of Berlin (9). His interest in this subject per- new method for illustrating the volumetric composition sisted unabated almost to the day of his retirement and, of ammonia (6) and was followed in 1865 by a lecture between 1870 and 1882, he continued to publish occa- to the London Chemical Society “On Lecture Illustra- sional updates (10). tions” (7). The many woodcuts used to illustrate the printed version of this lecture were obviously taken The Sodium Spoon from the plates for his forthcoming book, Introduction to Modern Chemistry Experimental and Theoretic, As revealed in his published lectures of 1866, Hof- which reproduced his introductory lectures at the mann’s approach to introductory chemistry was to Royal College of Chemistry and which was published gradually introduce his students to basic chemical con- the next year (8). He continued to refine and expand cepts in the course of experimentally demonstrating to his repertoire of lecture demonstrations after his return them the composition of three simple inorganic com- to Germany and, in 1869, during his tenure as Presi- pounds: hydrogen chloride, water, and ammonia (8).

Museum Notes, March/April 2013 1 WILLIAM B. JENSEN

strate their quantitative compositions volumetrically via electrolysis of their aqueous solutions:

Electricity + 2HCl(aq) ! H2(g) + Cl2(g) [2]

Electricity + 2H2O(l) ! 2H2(g) + O2(g) [3]

Electricity + 2NH3(aq) ! 3H2(g) + N2(g) [4]

Figure 2. Hofmann’s sodium spoon (right) as depicted in his published lectures of 1866.

He began in his first lecture by demonstrating their qualitative compositions, starting with the decomposi- tion of water using sodium metal: Figure 4. Hofmann’s V-cell for electrolysis as depicted in his lectures of 1866. It is powered by two Bunsen carbon cells.

2Na(s) + 2H2O(l) ! 2NaOH(aq) + H2(g) [1]

Rather than letting a piece of sodium zip around ran- domly on the surface of a dish of water, as is so often done today, Hofmann needed to collect the gas being formed in order to properly demonstrate that it was indeed dihydrogen gas. To do this, he developed a wire mesh spoon, known as a Hofmann sodium spoon (fig- ures 2-3), which allowed him to confine the reacting sodium beneath the mouth of an inverted cylinder of water in which he could collect the evolving gas by displacement and subsequently test it using a burning splint.

Electrolysis Apparatus

Having demonstrated the qualitative composition of his three compounds in Lecture I, Hofmann then pro- ceeded in Lecture II to further demonstrate their quan- titative compositions. Doing this gravimetrically would have been too time consuming and would not have lent itself to any easy visualization on the part of his audi- ence. Consequently, Hofmann chose instead to demon- Figure 5. A later version of the Hofmann V-cell for elec- trolysis from the Jensen-Thomas Apparatus Collection. It is being powered by a Grenet dichromate cell.

This approach also allowed him to introduce a discus- sion of both the ideal gas law and what we would to- day recognize as Avogadro’s hypothesis. The resulting Figure 3. A later version of the Hofmann sodium spoon from 1:1, 2:1, and 3:1 volume ratios further allowed him to the Jensen-Thomas Apparatus Collection. gradually introduce the newly emerging concept of

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atomic “quantivalence” or valence. For most of his electrolysis demonstrations Hof- mann made use of a V-shaped electrolysis cell (figures 4-5) powered by two Bunsen carbon batteries. How- ever, in a later lecture he also introduced a U-shaped cell with a central fill tube. By 1869 this came with an attached addition funnel (figures 6-7) and he had also introduced yet a third variation in the form of an H- shaped electrolysis cell (figures 6, 8). This latter form is the version which has survived today and which is most commonly associated with his name.

Figure 6. Hofmann’s alternative U- (center) and H-cells (left and right) for electrolysis as depicted in his lecture of 1869 to the Deutsche chemische Gesellschaft.

Figure 8. A later version of the Hofmann H-cell for elec- trolysis from the Jensen-Thomas Apparatus Collection with carbon rather than platinum electrodes. It is being powered by a Grenet dichromate cell.

Eudiometric Apparatus

Having demonstrated the composition of his three compounds via analysis, Hofmann then proceeded in Lecture III to confirm his results via their volumetric synthesis using the gas-gas reactions:

Figure 7. A later version of the Hofmann U-cell for elec- H2(g) + Cl2(g) → 2HCl(g) [5] trolysis from the Jensen-Thomas Apparatus Collection based on his final design of 1869. 2H2(g) + O2(g) → 2H2O(g) [6]

Museum Notes, March/April 2013 3 WILLIAM B. JENSEN

3H2(g) + N2(g) ! 2NH3(g) [7]

For this purpose he developed a piece of apparatus that was a cross between an eudiometer and a manometer (figures 9-10). A mixture of the reacting gases was added to the right arm of the U-tube through the glass stopcock and the changes in the mercury levels duly noted. The mixture was then sparked to initiate the

Figure 9. Hofmann’s apparatus for demonstrating the volume changes in gas-gas reactions as depicted in his 1869 lecture to the Deutsche chemische Gesellschaft. The necessary activation energy was provided via a spark gap and an induc- tion coil powered by two Bunsen carbon cells.

Figure 9. Hofmann’s apparatus for demonstrating the vol- ume changes in solid-gas reactions as shown in his lecture of 1869 to the Deutsche chemische Gesellschaft.

reaction using the embedded platinum wires and an induction coil. Changes in the mercury levels then re- vealed whether there was a net decrease, increase, or no change in the total volume of the gases as a result of the reaction. In 1869 Hofmann introduced a variation of this apparatus designed to demonstrate the volume changes accompanying gas-solid reactions, such as those be- tween dioxygen gas and either solid carbon or sulfur:

C(s) + O2(g) ! CO2(g) [8]

S(s) + O2(g) ! SO2(g) [9]

Figure 8. A later version of a Hofmann eudiometer from The solid in question was placed in a small metal the Jensen-Thomas Apparatus Collection used for demon- spoon located in the bulb at the top of the right arm of strating the volume changes in gas-gas reactions. the apparatus. The handle of this spoon was actually a

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These variations hardly exhaust the list of Hof- mann’s demonstration innovations and by 1912 the laboratory supply catalog for the American firm of Eimer and Amend was listing over 34 items under the heading of “Hofmann Lecture Apparatus,” some of which were mentioned in chemistry texts as late as 1939 (11, 12). However, with the exception of the H- shaped electrolysis cell, all of these have long since disappeared from the modern chemistry lecture hall.

Reference and Notes

1. J. J. Beer, “A. W. Hofmann and the Founding of the Royal College of Chemistry,” J. Chem. Educ., 1960, 37, 248- 251. 2. B. Lepsius, Festschrift zur Frier des 50 jährigen Bestehens der Deutschen Chemischen Gesellschaft und des 100. Geburtstages ihres Begründers August Wilhelm von Hofmann, Friedländer & Sohn: Berlin, 1918, pp. 51-54. 3. J. Volhart, E. Fischer, August Wilhelm Hofmann: Ein Lebensbild, Friedländer & Sohn: Berlin, 1902. For coverage of Hofmann’s contributions to lecture demonstrations, see pp. 280-284. 4. W. B. Jensen, “Reinventing the Hofmann Sodium Spoon,” Bull. Hist. Chem., 1990, 7, 38-39. 5. W. B. Jensen, “A. W. Hofmann and the Hofmann Electrolysis Apparatus,” Conference on Chemical Education, Figure 10. A later example of a Hofmann eudiometer from Butler University, Indianapolis, IN, 03 August 1993. the Jensen-Thomas Apparatus Collection used for demon- 6. A. W. Hofmann, “Veranschaulichung der volumetri- strating volume changes in solid-gas reactions. The glass schen Constitution des Ammoniaks,” Ann. Chem., 1860, 115, stopper at the top of the bulb is missing. 283-285. 7. A. W. Hofmann, “On Lecture Illustrations,” J. stiff metal wire that exited the top of the bulb and was Chem. Soc., 1865, 18, 156-172. connected to one terminal of a battery. A second wire 8. A. W. Hofmann, Introduction to Modern Chemistry was placed slightly above the spoon and connected to Experimental and Theoretic, Walton and Maberly, London, 1866. the latter via a fine platinum wire which rested on the 9. A. W. Hofmann, “Eine Vorlesung über Vorlesungs- spoon beneath the solid in question. Upon connecting versuche,” Berichte, 1869, 2, 237-268. this second wire to the battery in order to complete the 10. A. W. Hofmann, “Vorlesungsversuche,” Berichte, circuit, the platinum wire was heated to incandescence 1870, 3, 258-266; 1871, 4, 200-205; 1874, 7, 530-535; 1879, and thus served to ignite the carbon or sulfur in the 12, 1119-1126; 1882, 15, 2656-2677. spoon. Whereas in the various electrolysis cells, the 11. Chemical Apparatus, Assay Goods, and Laboratory electrical energy was used to drive a series of other- Supplies, Eimer & Amend: New York, NY, 1912, pp. 237-242, wise thermodynamically unfavorable reactions uphill, 349. in the various eudiometers it served instead as a source 12. J. R. Partington, A Textbook of Inorganic Chemis- of activation energy to initiate a series of kinetically inert, try, 5th ed., Macmillan: London, 1939, p. 501. but otherwise thermodynamically favorable reactions.

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